Magnetic incommensurability in pp-type cuprate perovskites

For the superconducting phase with a d-wave order parameter and zero temperature the magnetic susceptibility of the t-J model is calculated using the Mori projection operator technique. Conditions for the appearance of an incommensurate magnetic response below the resonance frequency are identified. A fast decay of the tails of the hole coherent peaks and a weak intensity of the hole incoherent continuum near the Fermi level are enough to produce an incommensurate response using different hole dispersions established for $p$-type cuprates, in which such response was observed. In this case, the nesting of the itinerant-electron theory or the charge modulation of the stripe theory is unnecessary for the incommensurability. The theory reproduces the hourglass dispersion of the susceptibility maxima with their location in the momentum space similar to that observed experimentally. The upper branch of the dispersion stems from the excitations of localized spins, while the lower one is due to the incommensurate maxima of their damping. The narrow and intensive resonance peak arises if the frequency of these excitations at the antiferromagnetic momentum lies below the edge of the two-fermion continuum; otherwise the maximum is broad and less intensive.Comment: 22 pages, 7 figure

The accessibility of research-based knowledge for nurses in United Kingdom acute care settings

Background. The successful dissemination of the results of the National Health Service (NHS) research and development strategy and the development of evidence based approaches to health care rely on clinicians having access to the best available evidence; evidence fit for the purpose of reducing the uncertainties associated with clinical decisions. Aim. To reveal the accessibility of those sources of information actually used by nurses, as well as those which they say they use. Design. Mixed method case site, using interview, observational, Q sort and documentary audit data in medical, surgical and coronary care units (CCUs) in three acute hospitals. Results. Three perspectives on accessibility were identified: (a) the humanist-in which human sources of information were the most accessible; (b) local information for local needs-in which locally produced resources were seen as the most accessible and (c) moving towards technology-in which information technology begins to be seen as accessible. Nurses' experience in a clinical specialty is positively associated with a perception that human sources such clinical nurse specialists, link nurses, doctors and experienced clinical colleagues are more accessible than text based sources. Clinical specialization is associated with different approaches to accessing research knowledge. Coronary care unit nurses were more likely perceive local guidelines, protocols and on-line databases as more accessible than their counterparts in general medical and surgical wards. Only a third of text-based resources available to nurses oil the wards had any explicit research base. These, and the remainder were Out of date (mean age of textbooks 11 years), and authorship hard to ascertain. Conclusion. A strategy to increase the use of research evidence by nurses should harness the influence of clinical nurse specialists, link nurses and those engaged in practice development. These roles Could act as 'conduits' through which research-based messages for practice, and information for clinical decision making, could flow. This role should be explored and enhanced

Acute care nurses' perceptions of barriers to using research information in clinical decision-making

Aim. To examine the barriers that nurses feel prevent them from using research in the decisions they make. Background. A sizeable research literature focusing on research utilization in nursing has developed over the past 20 years. However, this literature is characterized by a number of weaknesses: self-reported utilization behaviour; poor response rates and small, nonrandom sampling strategies. Design. Cross-case analysis involving anonymised qualitative interviews, observation, documentary audit and Q methodological modelling of shared subjectivities amongst nurses. The case sites were three large acute hospitals in the north of England. One hundred and eight nurses were interviewed, 61 of whom were also observed for a total of 180 h, and 122 nurses were involved in the Q modelling exercise (response rate of 64%). Results. Four perspectives were isolated that encompassed the characteristics associated with barriers to research use. These related to the individual, organization, nature of research information itself and environment. Nurses clustered around four main perspectives on the barriers to research use: (1) Problems in interpreting and using research products, which were seen as too complex, 'academic' and overly statistical; (2) Nurses who felt confident with research-based information perceived a lack of organizational support as a significant block; (3) Many nurses felt that researchers and research products lack clinical credibility and that they fail to offer the desired level of clinical direction; (4) Some nurses lacked the skills and, to a lesser degree, the motivation to use research themselves. These individuals liked research messages passed on to them by a third party and sought to foster others' involvement in research-based practice, rather than becoming directly involved themselves. Conclusions. Rejection of research knowledge is not a barrier to its application. Rather, the presentation and management of research knowledge in the workplace represent significant challenges for clinicians, policy-makers and the research community

Ordered and periodic chaos of the bounded one dimensinal multibarrier potential

Numerical analysis indicates that there exists an unexpected new ordered chaos for the bounded one-dimensional multibarrier potential. For certain values of the number of barriers, repeated identical forms (periods) of the wavepackets result upon passing through the multibarrier potential.Comment: 16 pages, 9 figures, 1 Table. Some former text removed and other introduce

Local electronic nematicity in the one-band Hubbard model

Nematicity is a well known property of liquid crystals and has been recently discussed in the context of strongly interacting electrons. An electronic nematic phase has been seen by many experiments in certain strongly correlated materials, in particular, in the pseudogap phase generic to many hole-doped cuprate superconductors. Recent measurements in high $T_c$ superconductors has shown even if the lattice is perfectly rotationally symmetric, the ground state can still have strongly nematic local properties. Our study of the two-dimensional Hubbard model provides strong support of the recent experimental results on local rotational $C_4$ symmetry breaking. The variational cluster approach is used here to show the possibility of an electronic nematic state and the proximity of the underlying symmetry-breaking ground state within the Hubbard model. We identify this nematic phase in the overdoped region and show that the local nematicity decreases with increasing electron filling. Our results also indicate that strong Coulomb interaction may drive the nematic phase into a phase similar to the stripe structure. The calculated spin (magnetic) correlation function in momentum space shows the effects resulting from real-space nematicity

The Optimal Inhomogeneity for Superconductivity: Finite Size Studies

We report the results of exact diagonalization studies of Hubbard models on a $4\times 4$ square lattice with periodic boundary conditions and various degrees and patterns of inhomogeneity, which are represented by inequivalent hopping integrals $t$ and $t^{\prime}$. We focus primarily on two patterns, the checkerboard and the striped cases, for a large range of values of the on-site repulsion $U$ and doped hole concentration, $x$. We present evidence that superconductivity is strongest for $U$ of order the bandwidth, and intermediate inhomogeneity, $0 <t^\prime< t$. The maximum value of the ``pair-binding energy'' we have found with purely repulsive interactions is $\Delta_{pb} = 0.32t$ for the checkerboard Hubbard model with $U=8t$ and $t^\prime = 0.5t$. Moreover, for near optimal values, our results are insensitive to changes in boundary conditions, suggesting that the correlation length is sufficiently short that finite size effects are already unimportant.Comment: 8 pages, 9 figures; minor revisions; more references adde

Theory of momentum resolved tunneling into a short quantum wire

Motivated by recent tunneling experiments in the parallel wire geometry, we calculate results for momentum resolved tunneling into a short one-dimensional wire, containing a small number of electrons. We derive some general theorems about the momentum dependence, and we carry out exact calculations for up to N=4 electrons in the final state, for a system with screened Coulomb interactions that models the situation of the experiments. We also investigate the limit of large $N$ using a Luttinger-liquid type analysis. We consider the low-density regime, where the system is close to the Wigner crystal limit, and where the energy scale for spin excitations can be much lower than for charge excitations, and we consider temperatures intermediate between the relevant spin energies and charge excitations, as well as temperatures below both energy scales.Comment: 19 pages, 13 figures, clarified text in a few points, added 1 figure, updated reference

Lanczos algorithm with Matrix Product States for dynamical correlation functions

The density-matrix renormalization group (DMRG) algorithm can be adapted to the calculation of dynamical correlation functions in various ways which all represent compromises between computational efficiency and physical accuracy. In this paper we reconsider the oldest approach based on a suitable Lanczos-generated approximate basis and implement it using matrix product states (MPS) for the representation of the basis states. The direct use of matrix product states combined with an ex-post reorthogonalization method allows to avoid several shortcomings of the original approach, namely the multi-targeting and the approximate representation of the Hamiltonian inherent in earlier Lanczos-method implementations in the DMRG framework, and to deal with the ghost problem of Lanczos methods, leading to a much better convergence of the spectral weights and poles. We present results for the dynamic spin structure factor of the spin-1/2 antiferromagnetic Heisenberg chain. A comparison to Bethe ansatz results in the thermodynamic limit reveals that the MPS-based Lanczos approach is much more accurate than earlier approaches at minor additional numerical cost.Comment: final version 11 pages, 11 figure

Randomised, controlled trial of alternating pressure mattresses compared with alternating pressure overlays for the prevention of pressure ulcers : PRESSURE (pressure relieving support surfaces) trial

Objective To compare whether differences exist between alternating pressure overlays and alternating pressure mattresses in the development of new pressure ulcers, healing of existing pressure ulcers, and patient acceptability. Design Pragmatic, open, multicentre, randomised controlled trial. Setting 11 hospitals in six NHS trusts. Participants 1972 people admitted to hospital as acute or elective patients. Interventions Participants were randomised to an alternating pressure mattress (n = 982) of- an alternating pressure overlay (n = 990). Main outcome measures The proportion of participants developing a new pressure ulcer of grade 2 or worse; time to development of new pressure ulcers; proportions of participants developing a new ulcer within 30 days; healing of existing pressure ulcers; and patient acceptability Results Intention to treat analysis found no difference in the proportions of participants developing a new pressure ulcer of grade 2 or worse (10.7% overlay patients, 10.3% mattress patients; difference 0.4%, 95% confidence interval - 23% to 3.1%, P = 0.75). More overlay patients requested change owing to dissatisfaction (23.3%) than mattress patients (18.9%, P = 0.02). Conclusion No difference was found between alternating pressure mattresses and alternating pressure overlays in the proportion of people who develop a pressure ulcer

High Speed Phase-Resolved 2-d UBV Photometry of the Crab pulsar

We report a phase-resolved photometric and morphological analysis of UBV data of the Crab pulsar obtained with the 2-d TRIFFID high speed optical photometer mounted on the Russian 6m telescope. By being able to accurately isolate the pulsar from the nebular background at an unprecedented temporal resolution (1 \mu s), the various light curve components were accurately fluxed via phase-resolved photometry. Within the $UBV$ range, our datasets are consistent with the existing trends reported elsewhere in the literature. In terms of flux and phase duration, both the peak Full Width Half Maxima and Half Width Half Maxima decrease as a function of photon energy. This is similarly the case for the flux associated with the bridge of emission. Power-law fits to the various light curve components are as follows; \alpha = 0.07 \pm 0.19 (peak 1), \alpha = -0.06 \pm 0.19 (peak 2) and \alpha = -0.44 \pm 0.19 (bridge) - the uncertainty here being dominated by the integrated CCD photometry used to independently reference the TRIFFID data. Temporally, the main peaks are coincident to \le 10 \mu s although an accurate phase lag with respect to the radio main peak is compromised by radio timing uncertainties. The plateau on the Crab's main peak was definitively determined to be \leq 55 \mu s in extent and may decrease as a function of photon energy. There is no evidence for non-stochastic activity over the light curves or within various phase regions, nor is there evidence of anything akin to the giant pulses noted in the radio. Finally, there is no evidence to support the existence of a reported 60 second modulation suggested to be as a consequence of free precession.Comment: 13 pages, 12 figures, accepted for publication in Astronomy & Astrophysic